1. Field of the Invention
This invention relates to a process for the deposition of films of cubic boron nitride and nitrides of other Group III elements in the periodic table of elements. More specifically, it relates to the use of the Activated Dissociation Reduction-Reaction process to produce coatings of cubic boron nitride on substrates for various applications.
2. Prior Art
Boron nitride exists in two allotropic forms--hexagonal and cubic. Cubic boron nitride films are useful in a variety of applications such as passivating and insulating layers for microelectronic applications, antireflection coatings for infrared optics, chemical and moisture resistant coatings, corrosion resistant coatings and hard coatings for cutting tools to produce higher cutting rates than those possible with tungsten carbide tools.
Several methods can be used to produce cubic boron nitride. At room temperature it is usually produced by a high pressure technology which is a complex, low-volume production method which makes the cutting tools so produced rather expensive. Synthesis of cubic boron nitride from boron nitride using certain aluminum alloys as catalysts is taught in U.S. Pat. No. 3,918,219, R. H. Wentrof, Jr., et al. This process is a high-temperature, high-pressure synthesis technique.
Alternate, inexpensive high-production methods for deposition of cubic boron nitride films involve vapor deposition techniques. A high-rate physical vapor deposition process called the Activated Reactive Evaporation process was invented by R. F. Bunshah, U.S. Pat. No. 3,791,852 for use with an electron-beam-heated evaporation source, and later by Bunshah and Nath, U.S. Pat. No. 4,336,277 for use with a resistance-heated source. Bunshah and Raghuram (Journal of Vacuum Science and Technology, Vol. 9, pp. 1385, 1972) deposited films of nitrides, carbides and oxides by the Activated Reactive Evaporation process using an electron beam-heated source. Bunshah and Nath deposited films of indium tin oxide using the resistance-heated evaporation source in the same process.
The resistance-heated evaporation source is preferred over the electron beam-heated source for low melting, high vapor pressure (i.e., easily evaporable) materials such as Indium, Tin, Zinc, etc. whereas the electron beam-heated source is preferred for evaporating high melting point metals such as titanium, zirconium, hafnium, niobium, etc.
Both versions of the Activated Reactive Evaporation process involve the same basic principle. Metal vapors are produced from a metal billet by heating the billet. A reactive gas at a low partial pressure (typically 2.times.10.sup.-4 to 1.times.10.sup.-3 torr) is introduced into the chamber. The metal vapor atoms and gas molecules are ionized by low energy electrons, thus "activating" the reaction between them, resulting in the formation and deposition of a compound film of nitrides, oxides or carbides on the substrate. Low-energy electrons (10 to 200 eV) are essential since their ionization cross-section is much higher than high energy electrons (above 500 eV). The only difference between the two versions of the activated reactive evaporation process is the source of the low-energy electrons to ionize the metal atoms and gas molecules. With the electron-beam evaporation source (U.S. Pat. No. 3,791,952) the low-energy electrons are extracted from the plasma sheath on top of the molten pool by means of a positively biased probe located some distance above the evaporation source and attracted into the space above the evaporation source and below the substrate to cause the ionization to occur. For the version using the resistance-heated source (U.S. Pat. No. 4,336,277), low-energy electrons emitted by an auxiliary thermionically heated tungsten filament traverse the path of the metal atoms and gas molecules to produce ionization. Figures illustrating both the apparatuses are given in the two patents referred to and are incorporated here by reference. Both processes are carried out in a vacuum chamber with appropriate equipment (pumps, valves, substarte heaters, etc.) as described in the patents cited above and are also incorporated here by reference. Typical examples of such reactions are: EQU Ti(atoms)+C.sub.2 H.sub.2 (gas).fwdarw.TiC(film)+H.sub.2 (gas) EQU 2Ti(atoms)+2NH.sub.3 (gas).fwdarw.2TiN(film)+3H.sub.2 (gas).
Beale (U.S. Pat. Nos. 4,297,387; 4,412,899; 4,415,420) uses the techniques of the Bunshah Pat. No. (3,791,852) to produce films of cubic boron nitride by evaporating boron or boron alloys from an electron beam-heated source in the presence of a reactive gas such as ammonia or nitrogen. He incorporates the Bunshah patent as part of his disclosure. Other features of the Beale patents are the use of selected alloying elements such as cobalt, nickel, manganese, zirconium, iron and aluminum in the billet, and thereby also in the cubic boron nitride film, to serve as "cubic phase nucleators". A low (400-700 C.) deposition temperature is used to reduce atomic motion and prevent the natural reversion of the cubic boron nitride to hexagonal boron nitride.
Experience with the evaporation process has shown that boron is brittle and very difficult to evaporate without cracking the boron evaporation billet due to the high thermal stresses produced by the large temperature gradient from the high temperature directly under the impact spot of the electron beam and the surrounding cooler metal. The impact spot of the electron beam is approximately 1/8 inch. diameter whereas the diameter of the billet is 1/2 to 3 inches. Further, it is very difficult and expensive to get billets of pure boron (or boron alloys) suitable for evaporation. For both of these reasons it is desirable to have an alternate, easily evaporable, and preferably cheap, source of boron.
The present invention is predicated on the use of boric acid (H.sub.3 BO.sub.3) as the evaporant. It is inexpensive, non-toxic, and easily evaporable from a relatively inexpensive resistance-heated evaporation source. The boric acid vapors react with ammonia gas in a plasma environment to deposit cubic boron nitride. The process consists of several consecutive steps as follows: EQU NH.sub.3 .fwdarw.3H+N (Dissociation) EQU H.sub.3 BO.sub.3 +3H.fwdarw.B+3H.sub.2 O (Reduction) EQU B+N.fwdarw.BN (Activated Reaction)
These steps lead to the name Activated Dissociation Reduction-Reaction process.
There are significant differences between the prior art and the present invention:
1. The prior art uses a boron billet for evaporation. The present invention uses an easily evaporable boron compound (boric acid, H.sub.3 BO.sub.3) which is reduced by the atomic hydrogen produced by the dissociation of ammonia gas in the plasma, the reduced boron then reacting with the atomic nitrogen present to form cubic boron nitride.
2. No catalysts or cubic phase nucleators are necessary.
3. The present process uses an inexpensive starting material (boric acid) compared to the expensive boron billet of the prior art.
4. The inexpensive boric acid can be readily evaporated from a cheap resistance-heated evaporation source in the present invention, compared to an expensive electron beam-heated evaporation source required to evaporate the boron in the prior art.